<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<style>
  /* .inner {
    width: 33.33%;
    height: 300px;
    display: inline-block;
    outline: solid 1px blue;
  } */

  /* 修改方法 */

  .inner {
    width: 33.33%;
    height: 300px;
    display: inline-block;
    outline: solid 1px blue;
    font-size: 30px;
  }

  .outer {
    font-size: 0;
  }
</style>

<body>

  <div class="outer">
    <div class="inner"></div>
    <div class="inner"></div>
    <div class="inner"></div>
  </div>

</body>

</html>